1. Filed of the Invention
This invention relates to a method for driving a charge-transfer type solid-state image pick-up device comprising high sensitivity photoelectric conversion elements for executing photoelectric conversion with relatively high sensitivity, low sensitivity photoelectric conversion elements for executing photoelectric conversion with relatively low sensitivity and vertical transfer channels for transferring signal charges from said high sensitivity photoelectric conversion elements and said low sensitivity photoelectric conversion elements, an image pick-up method using this image pick-up device and an image pick-up apparatus using this image pick-up device.
2. Description of the Related Art
A charge-transfer type solid-state image pick-up device transfers a signal charge stored in each of photoelectric conversion elements according to a quantity of incident light to vertical transfer channels arranged adjacently to the photoelectric conversion elements, sequentially transfers the signal charges to a horizontal transfer channel through the vertical transfer channels and sequentially transfers the signal charges to a floating diffusion amplifier through the horizontal transfer channel so that a voltage signal corresponding to the signal charge is produced. In such a solid-state image pick-up device, if there is a high luminance region on an image pick-up screen, a smear phenomenon is likely to occur. This leads to deterioration in image quality. In order to suppress this phenomenon, prior to the transfer of the signal charge to the vertical transfer channels, a high speed sweeping operation of transferring the smear charge remaining on the vertical transfer channels at a high speed is carried out. The method for driving the high speed sweeping operation whenever the charge is read out is described in JP-A-7-322148.
Further, where the different image signals with high sensitivity and low sensitivity acquired with different exposure times are composed to expand the dynamic range, with the image signal with the low sensitivity being non-linearly amplified with the smear signal of the polarity opposite to that of the image signal with the high sensitivity, the image signals with the low sensitivity and the high sensitivity are added/composed so that the smear components cancel each other. Such a smear correcting method is described in JP-A-8-37626.
However, in the method described in JP-A-7-322148, since the high speed sweeping operation of the smear charge is carried out whenever the signal charge is read out, the read time taken for reading the entire image is lengthened. This may lead to a hitch for the high speed read-out such as a successive shot or snap shot. Further, in the method disclosed in JP-A-8-37626, the driving system must be switched between the cases of the high sensitivity and the low sensitivity. This makes the control required therefor complicate.
Meanwhile, in the solid-state image pick-up device employed in an image pick-up apparatus such as a digital camera, the charge corresponding to an image signal is detected by a photoelectric conversion element (hereinafter also referred to as a “pixel”) so that it is generally difficult to extend the dynamic range. In order to obviate such an inconvenience, it has been proposed to obtain the image with a wide dynamic range by using the solid-state image pick-up device having photoelectric conversion elements with relatively high sensitivity (hereinafter also referred to as a “high sensitivity pixels”) and other photoelectric conversion elements with relatively low sensitivity (hereinafter referred to as a “low sensitivity pixels”) thereby to compose the high sensitivity image data from each the high sensitivity pixels and the low sensitivity image data from each the low sensitivity pixels.
The timing chart for the driving method in this case is shown in FIG. 7. As shown in FIG. 7, even in this configuration, where the signal charges from the high sensitivity and low sensitivity pixels are read out after exposure in the photoelectric conversion elements, prior to this reading, the high speed sweeping operation is carried out to remove the smear charges remaining on the vertical transfer channels.
As described above, in either driving method, in image pick-up of a still image, if the high speed sweeping operation is carried out after a shutter is closed, the time Tb taken for reading the entire image is lengthened. Thus, for the successive shot or snap shot in which the solid-state image pick-up device is caused to perform a high speed sequential image pick-up operation, the interval between image pick-up operations cannot be sufficiently shortened. Further, in image pick-up of a moving image, if the high speed sweeping operation for removing the smear component is not performed, a field step may occur to increase the possibility of generating a flicker so that FPN (Fixed Pattern Noise) appears on the screen.